Dude, we are all here applying via Express Entry.
If you get an ITA directly through rounds of invitation, then at the time of submitting e-APR, only your son will require the birth certificate.
How many CRS Points do you have?? Do you have more than 460 CRS Points?? If yes, then only your son will need a birth certificate.
If you have a score less than 450 CRS points, then you need to apply for a PNP to get additional 600 CRS points to get you the ITA. In that case - you, your spouse and your son - all of them will need the birth certificate.